(1) Field of the Invention
The present invention relates to a method for heat-treating a tube-shaped impact beam for reinforcing an automotive vehicle door to ensure the safety of a driver and a passenger in side collision and a system therefor. More particularly, it relates to a heat treatment of an impact beam mounted inside an automotive vehicle door and capable of meeting stringent mechanical property requirements such as tensile strength, yield strength and elongation without adding excessive weight to the vehicle.
(2) Description of the Prior Art
Competitively, automobile companies today in the world are typically to provide metal reinforcement to the inside of a panel of an automotive vehicle door in order to ensure the safety of the vehicle.
The reinforcement of the automotive vehicle door should have high strength, ductility and toughness without bending or being breaking against any impact to adversely affect a passenger's body. The mechanical characteristics and manufacturing processes should be designed and raw materials therefor be selected in order that the reinforcement is produced and supplied to the vehicle with minimum expense.
Since there may be a local buckling phenomenon according to a wall thickness-to-outer diameter ratio, the t/D ratio of the impact beam in a case that loads more than a predetermined [size] force is applied to the beam, what is important in such an impact beam, is defining the size range of materials that this phenomenon occurs or selecting a steel having a strength enough to meet mechanical property requirements required for use in the automobile industry.
For example, if a span L of a bending load applied to the steel tube is 800 mm, local buckling occurs in the range of t/D.ltoreq.8.5%. Therefore, if the outer diameter D is 31.8 mm, the thickness t should be more than 2.7 mm.
The increase in the thickness may prevent the local buckling of the beam but increases in the vehicle weight. Therefore, it is necessary to enhance mechanical properties to minimize the required thickness.
The automobile companies in the many countries have regulated prevailing mechanical properties of an impact beam to have a minimum tensile strength of about 110 kg/mm.sup.2 a minimum yield strength of about 80 kg/mm.sup.2 and a minimum elongation of about 10% pass the US Federal Motor Vehicle Safety Standard (FMVSS) regulation No. 214. However, an impact beam having more excellent mechanical properties than the above is required in order to reduce the weight of an automotive vehicle.
Recently, many techniques for manufacturing an impact beam have been described.
Representative conventional techniques are disclosed in the Korean Patent Publication No. 92-529 entitled "A method for making a tubing steel alloy to protect an automotive vehicle door" and in the Japanese Unexamined Patent Publication No. 4-52254 entitled "A method for making a quenched steel tube for reinforcement of an automotive vehicle door".
Another technique is disclosed in the Korean Unexamined Patent Publication No. 93-17645 entitled "A method for making an impact beam of an automobile" published on Sep. 20, 1993.
The above Korean Patent Publication No. 92-529 relates to a method for making a tubing steel alloy, a reinforcement not necessitating heat treatment, and discloses the chemical components of a steel alloy and a technique of hot rolling processing with a final rolling temperature of 980.degree. C. to 1080.degree. C.
The Japanese Unexamined Patent Publication No. 92-52254 relates to a method for making a reinforcing quenched tube of an automobile door having a high tensile strength with carbon steel containing about 0.1 to 0.3%carbon after high frequency induction heating and then water cooling.
This technique may achieve a high strength impact beam but has problems of poor weldability, susceptibility to cracking when roll-forming welding or using at low temperatures because of the relatively high carbon steel , and of mounting such an impact beam on a vehicle.
The Korean Unexamined Patent Publication No. 93-17645 discloses a 2-step heating treatment of firstly heating a welded tube for 5 to 150 seconds at 860.degree. to 950.degree. C., air-cooling the tube for 1 to 6 seconds and water-cooling the tube, and of secondly heating the tube for 5 to 15 minutes at 100.degree. to 250.degree. C. and air-cooling the tube, to achieve an impact beam having an excellent mechanical property of 130 kg/mm.sup.2 tensile strength. However, this technique has problems in the performance of systematical and effective heat treatment.
Besides, in line with the remarkable progress in the automotive vehicle industry today, it is necessary to comply with stringent mechanical property requirements, and to provide a more stable heat treatment process and system at minimum expense.